Antibody-drug conjugates (ADCs) are a class of therapeutics that combines the specificity of monoclonal antibodies (mAbs) with the potency of cytotoxic molecules. ADCs take advantage of characteristics of both components and significantly expand the therapeutic index of cytotoxic molecules by minimizing systemic exposure and associated toxicity while at the same time maximizing delivery of the cytotoxic agents to the target lesion, thus increasing treatment efficacy. Brentuximab Vedotin (SGN-35), an anti-CD30 antibody conjugated with cytotoxic agent MMAE, is already approved to treat CD30-positive relapsing lymphoma.
Target antigen selection, internalization of ADCs by tumor cells, and potency of cytotoxic drugs are parameters for ADC development (Carter 2008, Teicher 2009). In additional, the design of chemical linkers to covalently bind these building blocks to form an ADC also plays a role in the development of the ADCs (Ducry 2010). For example, the linker should be stable in the bloodstream to limit the damage to healthy tissue. Decomposition or decay of ADCs can release the cytotoxic drug before its delivery to the target sites. However, once the ADCs reach the target sites, they have to release the cytotoxic drug efficiently in its active form. The balance between plasma stability and efficient drug release at the target cell has yet to be found, which can depend on the linker design.
At least three types of linkers are applied in ADC design, namely, chemically-labile linkers, enzyme-labile linkers, and non-cleavable linkers (Ducry 2010). For chemically labile linkers, such as hydrazone linker for Mylotarg and disulfide-bearing 4-mercaptopentanoate linker for DM1/DM4, selective cleavage of the linker and payload release for ADC is based upon the differential properties of the linker between the plasma and some cytoplasmic compartment. Linkers are relative stable in the blood's neutral pH environment but can get cleaved once the ADC enters the lower pH environment inside the cell. An in vivo trial demonstrated that chemically-labile linkers often suffer from limited plasma stability.
Enzyme-labile linkers take an alternative approach—the differential activities of proteases inside and outside of the cells—to achieve control of the drug release. Proteases normally are not active outside cells due to the unfavorable pH conditions and the presence of serum protease inhibitors. A drug can be conjugated to antibody via peptide bond. The drug can be specifically cleaved from the antibody by the action of lysosomal proteases present inside the cells, and at elevated levels in certain tumor types (Koblinsk et al). Compared to ADC with chemically-labile linker, enzyme-labile linkers can achieve better control of the drug release. However, the increased associated hydrophobicity of some enzyme-labile linkers can lead to aggregation of ADC, particularly with strongly hydrophobic drugs.
A third class of linkers is non-cleavable linkers. The release of the drug is believed to occur via the internalization of the ADC followed by the degradation of the antibody component in the lysosome, resulting in the release of the drug which is still attached to the linker. These non-cleavable linkers are stable in serum, but compared to enzyme-labile linkers, no bystander effect can result due to the fact that the released drugs are charged and are not able to diffuse into neighboring cells. Also, since internalization of the ADC is a factor for the release of the drug, the efficacy is antigen- (and thus antibody-) dependent.
Linker technology affects ADC potency, specificity, and safety. There is a need for linkers for ADCs which can provide serum stability as well as increased solubility, allowing efficient conjugation and intracellular delivery of hydrophobic drugs.